Elevating Sludge Process Design: The Significance of Plant-Specific Rheology Data

RELEVANCE Wastewater sludges have fluid characteristics that differ from clean water and exhibit non-Newtonian behaviors that require specific hydraulic analyses. Rheology, the study of matter deformation and flow, can quantify rheological characteristics and improve the hydraulic design of wastewater sludge systems. A solids improvement project at Kitsap County's Central Kitsap Treatment Plant (CKTP) used onsite testing and hydraulic modeling to design various thickened sludge pumps and systems. The results from this design approach support using plant-specific rheological data when designing thickened sludge systems compared to conventional design approaches. BACKGROUND Common approaches to size sludge systems do not use plant-specific rheological testing data. Standard rheological values are typically used if plant data cannot be collected. However, this method usually only applies to systems under 100 feet in pipe length. Pump testing is another design approach but requires a constructed pump and sludge system, which must have been installed beforehand. Alternatively, a rotational rheometer can estimate sludge hydraulics by quantifying plant-specific rheological properties to develop a non-Newtonian hydraulic model. Rotational rheometers use a rotating spindle to apply shear force and torque to a fluid. A viscosity and shear stress profile can be generated by varying spindle speed to characterize the rheological properties. The solids concentrations of samples are also recorded to correlate the rheological profile with a specific percent total solids. These profiles can then be input into AFT Fathom, a hydraulic modeling software, to develop non-Newtonian hydraulic models. The Power Law, Bingham-Plastic, and Herschel-Buckly fits can be used to determine total dynamic head (TDH) for thickened sludge pumps. This hydraulic model can be used to design new thickened sludge systems or evaluate the impacts of changes to existing systems. CASE STUDY HDR is designing various solids process improvements at CKTP. New primary sludge, septage, and waste-activated sludge (WAS) thickening equipment and associated pumps, pipes, and appurtenances will be designed as part of the improvements. Rheological data collected from the plant was used to design new systems and ensure the adequacy of existing systems for process modifications. Onsite analysis of co-thickened primary sludge and septage, digested sludge, and thickened WAS were conducted using the rotational rheometer. Triplicate runs were run at various shear rates to measure each stream. In parallel, plant staff tested the total and volatile solids using standard methods. Rheological data was input into the hydraulic model, and various rheological fit curves were tested to determine design criteria for the thickened sludge systems. RESULTS Test results suggest a positive correlation between shear stress and total solids; viscosity measurement increased with colder sludge temperatures, resulting in more conservative design criteria. Figure 1 depicts shear rate and shear stress results for thickened waste activated sludge (TWAS), thickened primary sludge and septage (TPSS), and digester sludge. Figure 2 shows the rheometer during a sampling event. Sampled sludges had a lower solids (4-6%) than the design condition (5-8%) so fit curves were extrapolated to estimate sludge properties. Traditional design methods differed in modeled results as compared to this alternative approach. Table 1 compares the TDH for typical and alternative modeled design approaches. APPLICABILITY Typical design approaches use multiplication factors applied to clean water hydraulics or extrapolated data from region-specific case studies, which can result in incorrectly sizing pumps, pipes, and associated processes. For example, typical design approaches with 4-5% solids content varied by 13% between the TDH values through Kitsap's system. Alternatively, TPSS and TWAS samples had a smaller variation in total solids (0.1%) but varied in evaluated TDH by 100%. This is shown in Table 2 and reflects the point shown in Figure 1 that rheological behavior is not fully represented by solids content. Other rheological testing conducted showed up to 76% difference in TDH between similar sludges from different plants. This is also shown in Table 2. Using an alternative design approach of measuring plant-specific characteristics characterizes sludges with much more detail than typical approaches. The alternative approach mitigates inaccurate assumptions that could result in inefficient design of sludge systems. Collecting data from various plants will allow the establishment of a database to evaluate trends and consistencies in different sludges. An internal best practice document is in development with Matt Higgins at Bucknell University to develop standard testing criteria, data analysis methods, and design approaches for future applications. This effort focused on municipal sludges but could also be applied to industrial sludges and other viscous streams.